1. Preheat oven to 325°.
2. Microwave chocolate and butter in a large microwave-safe bowl at HIGH 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in sugar and eggs until well blended. Stir in flour. Spread half of batter into a greased 13- x 9-inch pan.
3. Beat softened cream cheese, 1/4 cup sugar, 1 egg yolk, and vanilla at medium speed with an electric mixer until smooth. Dollop cream cheese mixture over brownie batter in pan. Dollop with remaining brownie batter, and swirl together using a paring knife.
4. Bake at 325° for 35 to 40 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Let cool 1 hour on a wire rack. Cut into squares.